The H-mode pedestal structure and its role on confinement in JET with a carbon and metal wall

Abstract

We present the pedestal structure, as determined from the high resolution Thomson scattering (HRTS) measurements, for a database of low and high triangularity (  0.22-0.39) 2.5MA, Type I ELMy H-mode JET plasmas after the installation of the new ITER-like Wall (JET-ILW). The database explores the effect of increasing deuterium fuelling and nitrogen seeding with a view to explain the observed changes in performance (edge and global). The low triangularity JET-ILW plasmas show no significant change in performance and pedestal structure with increasing gas dosing. These results are in good agreement with EPED1 predictions. At high triangularity, for pure deuterium fuelled JET-ILW plasmas, there is a 20-30% reduction in global performance and pressure pedestal height in comparison to JET-C plasmas. This reduction in performance is primarily due to a degradation of the temperature pedestal height. The global performance and pressure pedestal height of JET-ILW plasmas can be partially recovered to that of JET-C plasmas with additional nitrogen seeding [Giroud 2013]. This observed improvement in performance is predominately due to a significant increase in density pedestal height as well as a small increase in the temperature pedestal height. A key result with increasing deuterium fuelling for JET-ILW plasmas is there is no improvement in pressure pedestal height however the pedestal still widens which is inconsistent with the  = 0.076√ pol,ped scaling. Furthermore, a key result with increasing nitrogen seeding is the pressure pedestal widening is due to an increase in the temperature pedestal width whilst the density pedestal shows no clear trend. The comparison of EPED1 predictions with the measurements at high triangularity is complex as, for example, for pure deuterium fuelled plasmas there is very good agreement for the pedestal height but not the width. In addition, current EPED1 runs under-predict the pedestal height and width at high nitrogen seeding for JET-ILW plasmas however further work is required to determine the significance of these deviations. Understanding these deviations is essential as provides an insight to the physical mechanisms governing the pedestal structure and edge performance.

Global and pedestal confinement and pedestal structure in dimensionless collisionality scans of low-triangularity H-mode plasmas in JET-ILW

“…It cannot be ascribed to the stability criterion used (γ > 0.03ω A in the present work) as discussed in [18]. The disagreement is, however, consistent with the earlier stability analysis in JET-ILW [4,18,58]. In particular, the results described in [18] in low-δ JET-ILW plasmas at I p = 1.4 MA B T = 1.7 T and ν * ped ≈ 0. show that the operational point is near the n = 70 boundary at low D 2 gas injection (Γ D2 ≈ 0.3 · 10 22 e/s) while it is in the stable region at higher gas (Γ D2 > 0.8 · 10 22 e/s).…”

Pedestal evolution physics in low triangularity JET tokamak discharges with ITER-like wall

“…Also the variations of the JET pedestal structure with collisionality, normalised Larmor radius, ρ * and normalised pressure, β N , are found to be qualitatively consistent with the peeling-ballooning mode stability constraints [19]. However, there are some trends that at first sight appear to be beyond the EPED model, such as the variation of pedestal height with strong gas puffing on JET [20] and the differences in pedestal structure between the carbon and ITER-like wall [18,21], as well as the impact of impurity seeding [22]. Furthermore, it is not always the case that the calculated peeling-ballooning stability boundary is reached at the onset of the ELM [23], and it is often the case that the JET pedestal width reduces between ELMs [24], while the pedestal height, and therefore β p,ped , increases.…”

Non-local effects on pedestal kinetic ballooning mode stability

“…However, this dependency does not seem to be universal, as a linear dependency between the width and the height (∆ ∼ β p,ped ) was observed in NSTX [8]. Also pedestal widening without a change in height has been observed with increased gas fuelling in JET with the Beryllium-Tungsten wall [9]. Therefore, relying on the empirical ∆ ∼ β p,ped scaling without understanding the physical basis of the scaling can lead to wrong answers especially if it is used for future devices operating outside the parameters used in current experiments.…”